Add trend visualization to GitHub Step Summary

[Mossaka]

Problem

Benchmark results are reported as a flat table of the current run. There's no way to see if metrics are trending up or down without manually comparing artifact downloads.

Approach

The benchmark-trend.ts script (from #1866) generates a Markdown trend table appended to $GITHUB_STEP_SUMMARY:

1. Latest vs Previous — table showing delta and % change per metric with directional indicators
2. Historical table — last 10 runs showing P95 for each metric over time

Example output:

### Latest vs Previous Run
| Metric | Previous P95 | Current P95 | Delta | % Change |
|--------|-------------|-------------|-------|----------|
| container_startup_warm | 18,469ms | 13,200ms | -5,269ms | -28.5% |

### Historical Results (P95)
| Date | Commit | Warm Start | Cold Start | HTTPS Latency | Memory |
|------|--------|------------|------------|---------------|--------|
| 2026-04-09 | 0be647f | 18,469ms | 26,160ms | 70ms | 20MB |

[Mossaka]

Part of benchmark epic #240. Depends on #1866 (historical storage).